This invention relates to Schottky diodes and to a novel process for their manufacture and more specifically relates to a novel process for the manufacture of a Schottky device in which the Schottky forming metal securely adheres to the surface of the device even though it does not adhere well to a peripheral oxide passivation coating.
Schottky diodes are well known which include a molybdenum electrode which forms a Schottky barrier on a silicon substrate. In order to operate such devices at relatively high blocking voltages, it is necessary to use a guard ring of opposite conductivity type to that of the main body, which guard ring is preferably contacted by the molybdenum Schottky forming electrode. It is also desirable to provide a system of solderable metals atop the molybdenum contact. Typically, a titanium or chromium barrier layer is first laid atop the molybdenum layer, followed by a layer of nickel and a layer of silver.
While many different metals can be used to form the schottky barrier, it is necessary to select a metal which strikes a good balance between the desired turn-on voltage of the device in the forward direction and the leakage current in the reverse direction. These are frequently conflicting goals.
Molybdenum is known to provide a good compromise to these contradictory requirements. However, when molybdenum is used in a high voltage device where it is desirable that the metal forming a Schottky contact overlay the guard ring oxide to produce a field plate effect, the molybdenum will not adhere well to the oxide surface. Consequently, devices employing molybdenum overlying the guard ring oxide have experienced failures due to peeling of the molybdenum and the generation of metal chips within the housing. These devices also have a reduced reverse energy handling capability due to poor contact to the guard ring. This has led to very poor yields in the manufacturing process and the yield for 100 volt devices has been as low as 2%.
Processes are known in which titanium or chromium layers are first applied atop the oxide which is atop the guard ring before a Schottky barrier metal is applied to the main exposed silicon surface of the device. After the oxide surface is coated with the chromium to titanium, a molybdenum or palladium layer is applied over the entire surface of the device, adhering well to both the underlying bare silicon and the titanium or chromium. A process of this type is disclosed, for example, in U.S. Pat. No. 3,642,528. This process is relatively complex and requires numerous masking steps and the process is of a nature which does not normally produce a good yield. Unavoidable back-plating of the titanium and chromium contaminates the desirable extremely clean silicon surface which receives the blocking layer.